The different IEEE 802.11a, b, g, n WiFi standards use transmission frequency channels that can be equal to a frequency width of 20 MHz to 40 MHz (40 MHz being a concatenation of two 20 MHz radiofrequency channels or frequency bands without overlapping), or can even allow transmission on an 80 MHz frequency band in concatenating four 20 MHz radiofrequency channels which may or may not be contiguous, in a future version of the IEEE 802.11n WiFi standard.
Frequently, several WiFi access points are located in the same frequency bands and in the same spatial environment. These are then said to be overlapping basic service sets. Thus, a radiofrequency channel or a frequency band has to be shared between the different basic service sets (BSSs).
This is done in the classic way by using the CSMA-CA (Carrier Sense Multiple Access-Collision Avoidance) mode, as described in the standard 802.11-2007, paragraph 9.1 “MAC architecture”, 9.1.1 “DCF”.
The CSMA-CA mechanism illustrated in FIG. 1 ensures a sharing of the access to a radiofrequency channel according to a principle known as that of contention: each device must listen to see whether the channel is free (i.e. that no signal is being sent/received in this channel) for a variable duration, corresponding to an arbitrary inter-frame duration known as the AIFS (“arbitration inter-frame space”) and a random waiting period (denoted as B for “backoff”) before transmitting data.
In practice, during the contention period, the access point decrements the backoffs for each packet waiting in the queues so long as the channel is free.
Should several radiofrequency channels be concatenated, the channel being listened to is called a primary channel referenced 1 in FIG. 1.
Should the first packet in the queue have to be transmitted on a 20 MHz frequency band and be therefore likely to gain access to the channel (if no channel takes uplink control of the channel before the end of the countdown), then only the primary channel is listened to (FIG. 1).
In the case of the future standard which will enable transmission on an 80 MHz frequency band in concatenating four radiofrequency channels, the new generation of devices, here below called “high throughput” or “HT” devices, i.e. devices capable of implementing this standard, can transmit on an 80 MHz frequency band.
By contrast, the former-generation devices (i.e. devices that implement earlier standards), here below called legacy devices, cannot transmit on an 80 MHz frequency band.
Now, for reasons of backward compatibility, the CSMA-CA mechanism enables each device, whether a legacy device or a new-generation device, to “take control” of a channel or a frequency band to make transmission at 20 MHz, 40 MHz or 80 MHz.
One drawback of this prior-art technique lies in the fact that if an access point takes control of a channel to transmit data to a legacy station limited for example to a 20 MHz frequency band (or to an HT station for a packet that does not require the use of more than 20 MHz), the remaining 60 MHz of the 80 MHz frequency band of the concatenated channel are not used. The spectral efficiency of the basic service set is severely reduced.
Similarly, a present-day technique known as a “20/40 mechanism” and defined in the IEEE 802.11n standard enabling transmission on a frequency band greater than 20 MHz, i.e. 40 MHz in this case, allows a device (a station or an access point) wishing to transmit data packets on a 40 MHz frequency band to a single user, known as the destination device (namely the access point or a station respectively), to reserve this 40 MHz frequency band. However this standard does not enable the transmission of data packets on a 40 MHz frequency band to several destination devices.
Finally, the OFDMA multiple-access technique enables the transmission of the data packets to two destination devices on an 80 MHz frequency band in using distinct carriers of this frequency band respectively for each of the destination devices and in informing each of the destination devices as to which carriers concern it. The destination devices must therefore be compatible with this technique, in order to recognize the signaling by which they are able to know which carriers concern them.
There is therefore a need for a novel technique enabling optimum use of an available frequency band to transmit data packets to different destination devices, whether of the old or the new generation, so as to optimize the spectral efficiency and the total throughput rate of a basic service set.